This invention relates generally to planar lightwave circuits for use in optical signal routing applications, in particular, planar lightwave circuits having arrayed waveguide gratings.
The increase in Internet traffic, the number of telephones, fax machines, computers with modems, and other telecommunications services and equipment over the past several years has caused researchers to explore new ways to increase fiber optic network capacity by carrying multiple data signals concurrently through telecommunications lines. To expand fiber network capacity, fairly complex optical components have already been developed for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM).
In a WDM system, multiple optical data signals of different wavelengths are added together in a device called a multiplexer and the resulting data signal is transmitted over a fiber optic cable. The wavelength division multiplexed signal comprises a plurality of optical signals having a predetermined nominal wavelength difference from each other. A demultiplexer separates the multiple optical data signals of different wavelength. Any WDM system must include at least one component to perform the function of optical multiplexing (namely, the multiplexer) and at least one component to perform the function of optical demultiplexing (namely, the demultiplexer). The optical multiplexer and the optical demultiplexer are each examples of optical wavelength routers.
In general, an optical wavelength router has at least one input optical port and at least on output optical port. In an optical router, light may be transmitted from a specific input port to a specific output port only if the light has an appropriate wavelength. Complex WDM systems may require optical wavelength router components that are more complex than a multiplexer or a demultiplexer. For example, an arrayed waveguide grating (AWG) or an integrated reflection grating may be used in a multiplexer, a demultiplexer, or a more general optical router.
Planar lightwave circuit technology is one technology that may be used to implement an optical wavelength router. A planar lightwave circuit (PLC) is an application of integrated optics. In a PLC, light is restricted to propagate in a region that is thin (typically between approximately 1 xcexcm and 30 xcexcm) in one dimension, referred to herein as the lateral dimension, and extended (typically between 1 mm and 100 mm) in the other two dimensions. A plane that is perpendicular to the lateral dimension of the PLC is defined as the plane of the PLC. The longitudinal direction is defined as the direction of propagation of light at any point on the PLC. The lateral direction is defined to be perpendicular to the plane of the PLC. The transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
In a typical example of a PLC, a slab waveguide comprises three layers of silica glass are used with the core layer lying between the top cladding layer and the bottom cladding layer. Channel waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. In this example, each layer is doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer. As a second example, slab waveguides and channel waveguides comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the slab waveguide and/or channel waveguide may comprise an optically transparent polymer. Another example of a slab waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
A PLC optical router comprises an optical waveguide for each input optical port and an optical waveguide for each output port. Each input and output optical waveguide confines the light in both the lateral and the transverse direction. A PLC optical router also comprises at least one region comprising a slab waveguide, which confines the light in the lateral direction but not in the transverse direction. A PLC optical router further comprises at least one optical dispersive region, which may be either an arrayed waveguide grating (AWG) region or an integrated reflection grating.
FIG. 1 depicts an AWG optical router that acts as a demultiplexer 10. A plurality of optical signals incident on one input optical port propagates through the device in the following sequence: the signals propagate through an input waveguide 12, which is a input waveguide associated with the input port; through an input slab waveguide 14, which has the function of expanding the optical field in the transverse direction by diffraction; through the dispersive region 16 (namely, the array waveguide region) comprising an array of AWG waveguides 18 for modifying the direction of propagation for each wavelength constituent according to the wavelength of the constituent of the plurality of signals; through an output slab waveguide 20 for focusing the signals of different wavelength coupled from the dispersive region 16 into a plurality of predetermined positions in accordance with the predetermined wavelength difference; through a plurality of output waveguides 22 each associated with one output port.
The dispersive property of the arrayed waveguide grating (AWG) region is attributable to the construction of the plurality of waveguides within the waveguide grating region such that adjacent waveguides have a predetermined length difference in accordance to the required dispersive properties of the dispersive region 16, so that each signal at different wavelength coupled to and traveling over each channel waveguide is provided with a phase difference from each other in accordance with the predetermined length difference. Each of the output waveguides 22 includes an input end 24, which is arranged at a predetermined position, so that each separated signal at each wavelength is coupled to each output waveguide 22 and emerges from an output end 26 thereof.
In operation, the wavelength division multiplexed signals coupled into the input channel waveguide 12 expand into the input slab waveguide 14 by diffraction. Then, the expanded signals are distributed to the channel waveguides 18 of the arrayed-waveguide grating 16. Because each channel waveguide 18 of the arrayed-waveguide grating 16 has a predetermined waveguide length difference, each signal, after traveling over each channel waveguide 18 to the output slab waveguide 20, has a predetermined phase difference according to its waveguide length difference. Since the phase difference depends on the wavelength of the signal, each signal at different wavelength is focused on a different position along the arc boundary 28 of the output slab waveguide 20. As a result, separated signals, each having a different wavelength, are received by the plurality of output channel waveguides 22 and emerge therefrom, respectively.
The general principles and performance of an AWG multiplexer are similar to the AWG demultiplexer, except that the direction of propagation of light is reversed, the ports that act as inputs for the demultiplexer act as output ports for the multiplexer, and the ports that act as output ports for the demultiplexer act as input ports for the multiplexer.
Multiple routing functions including multiplexing and demultiplexing may be integrated on a silicon wafer to form a complex planar lightwave circuit (PLC). PLCs can be made using tools and techniques developed to extremely high levels by the semiconductor industry. Integrating multiple components on a PLC may reduce the manufacturing, packaging, and assembly costs per function.
The details of the design and manufacture of an optical router comprising an AWG depend to some extent on the performance requirements. Aspects of performance that are affected by the present invention are referred to as insertion loss, passband width, ripple, and adjacent channel isolation. These terms, as well as a number of related terms will now be defined.
Spectral transmissivity (in units of dB) is defined as the optical power (in units of dBm) of substantially monochromatic light that emerges from the fiber that is coupled to the input port minus the optical power (in units of dBm) of the light that enters the optical fiber that is coupled to the output port of the optical router. Spectral transmissivity is a function of the selected input port, the selected output port, the optical wavelength, and the polarization state of the incident light. As illustrated, for example, in FIG. 2a, the maximal spectral transmissivity 30 refers to the spectral transmissivity for the incident polarization state that provides the maximum value for spectral transmissivity. The minimal spectral transmissivity 32 refers to the spectral transmissivity for the incident polarization state that provides the minimal value for spectral transmissivity. In general, the incident polarization state used to determine maximal and minimal spectral transmissivities is a function of wavelength, and depends on the input port and the output port used. The mean spectral transmissivity (in dB units) 34 is defined as the mean of the maximal spectral transmissivity (in dB units) 30 and the minimal spectral transmissivity (in dB units) 32.
Insertion loss (IL) is illustrated in FIG. 2b. The insertion loss for a particular input/output port combination is defined as the minimum value 36 of the minimal spectral transmissivity 38 within the International Telecommunication Union (ITU) band 40 (i.e., a 0.2 nm range of wavelengths that is centered on a predetermined target wavelength) for the particular input/output port combination. The center wavelength (CW) for a particular input/output port combination is defined as the mean value of all wavelengths of light that provide a mean spectral transmissivity that is larger than xe2x88x923 dB.
Ripple is illustrated with reference to FIGS. 2c and 2d. Ripple (in dB units) for a particular input/output combination is defined as the maximum value 42 of the maximal spectral transmissivity 44 within the ITU band 46 minus the minimum value 48 of the minimal spectral sensitivity 50 within the ITU band 46. For example, FIG. 2c corresponds to a relatively small disparity in taper widths and FIG. 2d corresponds to a relatively large disparity in taper widths.
The passband width depends on a predetermined reference insertion loss level and the particular input/output combination. Of particular interest is the value denoted as xe2x80x9cxe2x88x921.0 BWxe2x80x9d, which is the passband width with respect to the xe2x88x921 dB insertion loss reference level. For example, as shown in FIG. 2e, xe2x88x921.0 BW is defined as the difference in wavelength between a first wavelength and a second wavelength, wherein both the first and the second wavelengths provide a mean spectral transmissivity that equals xe2x88x921.0 dB for a particular input/output port combination, and the first wavelength is larger than the center wavelength (CW) and the second wavelength is smaller than the center wavelength. The definitions of xe2x88x920.5 BW, xe2x88x923.0BW and xe2x88x9220BW are the same as xe2x88x921.0BW, except that the spectral transmissivity reference levels are xe2x88x920.5 dB, xe2x88x923.0 dB and xe2x88x9220 dB respectively.
Adjacent isolation (ADJ_ISO) is illustrated in FIG. 2f. For a multiplexer, ADJ_ISO is defined as the difference between a first spectral transmissivity 52 and a second spectral transmissivity 54, wherein the first spectral transmissivity is the minimum 52 of the minimal spectral transmissivity 54 within the ITU band 58 associated with a predetermined first input/output port combination and the second spectral transmissivity is the maximum 54 of the maximal spectral transmissivity 60 associated with a second input/output port combination, wherein the first and second input/output port combinations share a common output port and the input ports of the first and second input/output ports combinations are adjacent. ADJ_ISO for a demultiplexer is defined in a similar manner, except that a common input port is used and adjacent output ports are used in the definition.
IL_AVE is defined as the average (AVE) insertion loss (IL) of values for all input/output port combinations that are used for a particular application of the device. RIPPLE_AVE, xe2x88x920.5BW_AVE, xe2x88x921.0BW_AVE, xe2x88x923.0BW_AVE, xe2x88x9220 BW_AVE, and ADJ_ISO_AVE, have similar definitions.
IL_WC is defined as the value of insertion loss (IL) for the input/output combination (selected from among those that are used for a particular application) that provides the xe2x80x9cworst casexe2x80x9d (WC) value of IL, i.e., the smallest IL value. RIPPLE_WC is defined as the value of RIPPLE for the input/output combination (selected from among those that are used for a particular application) that provides xe2x80x9cworst casexe2x80x9d value of RIPPLE, i.e., the largest value of RIPPLE.
The performance requirements depend to some extent on the type of AWG optical router. For example, the performance requirements for a multiplexer differ to some extent from the performance requirements of a demultiplexer. High adjacent channel isolation (i.e. a small value for ADJ_ISO) is critical for a demultiplexer, but of no consequence for a multiplexer. A low insertion loss (i.e., a high value for IL), a wide passband width and low ripple are desirable for both a multiplexer and a demultiplexer; however, the design changes to achieve each of these individually may be contrary to the design requirements imposed by other performance requirements. For example, a design change to widen the passband may increase insertion loss (i.e., reduce the IL value). As a second example, a design change to widen the passband may reduce adjacent channel isolation (i.e., increase ADJ_ISO). This second example is important for a demultiplexer but is of no significance for a multiplexer.
To the extent that other performance parameters are not adversely affected, it is desirable to have a wide passband width for a number of reasons. For example, in an optical network, a signal may originate from many different transmitters and then travel through many routers. Each of the transmitting lasers emitting at a channel wavelength must transmit within a given fraction of the allotted bandwidth. However, these lasers tend to drift for a number of reasons including variation in ambient temperature, aging, and other reasons. A wider passband width having a uniform insertion loss across the passband allows the lasers to drift without significantly affecting system performance. Also, a wider passband width generally reduces ripple within one channel.
The passband width depends to a large extent on the details of the design in two regions of the AWG-based optical router: the region where the input waveguide joins the input slab waveguide, and the region where the output waveguide joins the output slab waveguide. In a conventional AWG-based optical router, the width of the waveguide at the point where it joins the slab waveguide determines the size of the fundamental mode of the input/output waveguide that is supported by the input/output waveguide at the point of transition between the slab waveguide and the input/output waveguide. As a general trend, increasing the size of the fundamental mode on either the input side or the output side increases the passband width; however, the general trend has exceptions and is complicated by the fact that a portion of the optical power may propagate in modes that are of higher order than the fundamental mode. To take advantage of the general trend, the prior art describes the application of a taper 62, as illustrated in FIG. 3, comprising a input waveguide 64 that substantially increases in width as it approaches the slab waveguide 66 to which it is attached. If the taper 62 is sufficiently wide, a portion of the optical power propagates in at least one mode in addition to the fundamental mode and complications may arise, which include the possibility of introducing a local minimum in the passband and thereby adversely affecting passband ripple along with the increase in passband width, for example, as depicted in FIG. 2d. A taper region provides a transition from a first input waveguide segment 64 that is optimized for its transmission properties to a second input waveguide segment 68 that is optimized for its control of the mode size at a point 70 where it joins the slab waveguide 66. The width of the input waveguide 64 at the point 70 of attachment to the slab waveguide 66 is referred to as the taper width.
In an AWG multiplexer, the plurality of input waveguides 64 is attached to the input slab waveguide along an arc that is limited in extent by design requirements. The limited extent of this region limits input taper width. The input taper pitch is defined as distance between the centers of two adjacent tapers where they meet the slab waveguide. In the conventional AWG multiplexer, input taper pitch together with the limits of the fabrication process limits the taper width and consequently limits the size of the fundamental mode at the end of the taper. The output taper width is usually less restricted. Consequently, on a multiplexer, the output taper width is usually wider than the input taper width. Similarly, on a demultiplexer, the input taper width is usually wider than the output taper width. Increasing either or both taper widths will increase the passband width; however, increasing the disparity between the widths on the input side and the output side adversely affects the insertion loss and may adversely affect passband ripple. For a multiplexer, it is desirable to maximize the width of the fundamental mode on the input side in order to maximize the passband width without adversely affecting the insertion loss and select the mode size on the output side that provides the best trade-off between insertion loss and passband width. For the conventional multiplexer, this line of reasoning implies that the input taper widths should be as large as possible within the limits imposed by the pitch of the input tapers and the fabrication limitations. It should be emphasized that this line of reasoning is not rigorous and may not be effective in practice or may cause other performance requirements to fail; ultimately the approach requires careful experimental verification. For some multiplexers, the output taper widths should also be as large as possible within the limits imposed by the pitch of the input tapers and the fabrication limitations. A typical fabrication process may impose a gap that is between approximately 1 xcexcm and 5 xcexcm, resulting in a maximum taper width that is less than the input taper width by an amount that is between 1 xcexcm and 5 xcexcm.
One objective of the present invention is to efficiently broaden the passband width of an AWG-based optical router using a taper region. In pursuit of this objective, a taper region is described that is designed to maximize the size of the optical mode at the end of the tapers within the constraints imposed by the taper pitch and fabrication limitations. When applying this to a demultiplexer, the extent to which the passband is broadened must be balanced against the reduction in adjacent channel isolation. When applying this to a multiplexer, the adverse affect on the adjacent channel isolation is not significant. Various embodiments of this invention address these issues.
In accordance with one aspect of the present invention, there is provided an optical wavelength router that includes at least one input waveguide, an input slab waveguide, an arrayed waveguide grating, an output slab waveguide, at least one output waveguide, and at least one dendritic taper region. The input slab waveguide is optically coupled to the at least one input waveguide. The input slab waveguide and an output slab waveguide are optically coupled via the arrayed waveguide grating. The at least one output waveguide is optically coupled to the output slab waveguide. The dendritic taper region includes at least one dendritic taper. The dendritic taper includes a trunk having a first end and a second end and at least one branch optically coupled to the trunk.
In accordance with another aspect of the present invention, there is provided an optical wavelength router that includes at least one input waveguide, an input slab waveguide, an arrayed waveguide grating, an output slab waveguide, at least one output waveguide, and at least one dendritic taper region. The input slab waveguide is optically coupled to the at least one input waveguide. The arrayed waveguide grating is optically coupled to the input slab waveguide. The output slab waveguide is optically coupled to the input slab waveguide via the arrayed waveguide grating. The at least one output waveguide is optically coupled to the output slab waveguide. The at least one dendritic taper region includes at least one dendritic taper which includes a trunk having a first end and a second end. The dendritic taper includes at least one branch optically coupled to the trunk. At least one of the dendritic taper regions is an input dendritic taper region located between the at least one input waveguide and the input slab waveguide. The input dendritic taper region is optically coupled to the input slab waveguide and to the at least one input waveguide. The first end of the trunk of the input dendritic taper region is located distally from the input slab waveguide relative to the second end of the input dendritic taper region which is located proximately to input slab waveguide.
In accordance with yet another aspect of the present invention, there is provided an optical wavelength router that includes at least one input waveguide, an input slab waveguide, an arrayed waveguide grating, an output slab waveguide, at least one output waveguide, and at least one dendritic taper region. The input slab waveguide is optically coupled to the at least one input waveguide. The arrayed waveguide grating is optically coupled to the input slab waveguide. The output slab waveguide is optically coupled to the input slab waveguide via the arrayed waveguide grating. The at least one output waveguide is optically coupled to the output slab waveguide. The at least one dendritic taper region includes at least one dendritic taper which includes a trunk having a first end and a second end. The dendritic taper includes at least one branch optically coupled to the trunk. At least one of the dendritic taper regions is an output dendritic taper region located between the at least one output waveguide and the output slab waveguide. The output dendritic taper region is optically coupled to the output slab waveguide and to the at least one output waveguide. The first end of the trunk of the output dendritic taper region is located distally from the output slab waveguide relative to the second end of the output dendritic taper region which is located proximately to output slab waveguide.
In accordance with another aspect of the present invention, there is provided an optical wavelength router that includes at least one input waveguide, a slab waveguide, at least one output waveguide, and at least one dendritic taper region. The slab waveguide is optically coupled to the at least one input waveguide and to the at least one output waveguide. The slab waveguide includes an integrated reflection grating. The at least one dendritic taper region includes at least one dendritic taper that includes including a trunk having a first end and a second end. The dendritic taper includes at least one branch optically coupled to the trunk.